I'd do it on a cool evening to save the buckets of sweat
+1 to that
One trick I learnt from watching the pro tour mechanics at the TDU is once you've got enough of the tire separated from the rim that you can get something round (like a screwdriver) between the rim and the tire, push it all the way through so you can grip both ends and then push or pull the screwdriver around the wheel to separate the glue. I've found this leaves a more uniform layer of glue on the rim than just yanking it off (but note, I do not advocate using the pointy end of a screwdriver to initiate the separation of tire and rim, you're just going to damage one or the other in the process).
If you're using the same type of glue and the existing layer is consistant, just layer over it like Blybo and Duckmeister have suggested. If you want/need to completely remove glue from a Zipp, they recommend a product called Goof Off which you should be able to get from Bunnings, it works better than acetone but won't destroy the resin used in the carbon rims. If you don't want a massive goopy mess, try to scrape off as much excess glue as possible before you start with the chemicals, I keep an old blunt butter knife in my tool kit for just such a reason, it's blunt enough to not damage the rim, round enough to get in to the rim bed and stiff enough to budge the old glue. Be prepared to spend a lot of time doing this, there's no way to cheat (but I'm open to suggestions)
The best trick I've found in stretching new tubs is to seal the valve and soak the base tape in a bucket of water before stretching it over an old rim to dry overnight. This works best with conti's as their base tape isn't coated like some vittorias. Think of what stretching a wet cotton T-shirt does to it's shape and you're headed in the right direction. I don't have any old tubular rims so I've always stretched tires on an old clincher and never had a problem, even with a layer of glue on them.